This invention relates to a communication method and system using code division multiple access (CDMA) and, in particular, to a CDMA communication method and a spread spectrum communication system adapted to a forward packet transmission.
In a packet transmission of a CDMA communication system, generally, a spreading code and a frequency band are reserved at a time when a packet to be transmitted occurs, communication is made using a reserved spreading code and a reserved frequency band, and the reserved spreading code and the reserved frequency band are released when the communication comes to end.
Such a technique is, for example, disclosed in Japanese Unexamined Patent Publication of Tokkai No. Hei 9-327,072 or JP-A 9-327,072 (which will be called a first publication). Specifically, the first publication discloses xe2x80x9cCDMA COMMUNICATION METHOD AND SPREAD SPECTRUM COMMUNICATION SYSTEMxe2x80x9d each of which is capable of enhancing the flexibility of communication service by processing information individually. A frequency band for a communication zone between a base station and a plurality of terminal apparatuses (mobile stations) are divided into a forward (downlink) band in a forward direction and a reverse (uplink) band in a reverse direction. In addition, a plurality of carrier frequencies are defined corresponding to a plurality of sub-frequency bands which has one of a plurality of kinds of bandwidths in both the frequency bands. A sub-frequency band having the bandwidth in matching with the information transmission speed is preliminarily assigned to each terminal apparatus. Each terminal apparatus spreads a spectrum of a reserved packet using a reservation channel spreading code to produce a spread spectrum reserved packet and then transmits the spread spectrum reserved packet by using a specific carrier frequency corresponding to the reverse sub-frequency band. After the base station spreads a spectrum of a reply packet using a reply channel spreading code to produce a spread spectrum reply packet, the base station transmits the spread spectrum reply packet by using a particular carrier frequency corresponding to the forward sub-frequency band. After each terminal apparatus spreads a spectrum of an information packet using a spreading code for an information transmission channel designated by the reply packet to produce a spread spectrum information signal, each terminal apparatus transmits the spread spectrum information signal by using a specific carrier frequency in the reverse direction.
In addition, in the CDMA communication system, control of transmission power is important. Accordingly, as regards a pair of reverse like (uplink) and forward link (downlink), power control information of a link for transmitting data is generally transmitted using a link in the opposite direction to the link for transmitting data.
However, problems arise in the above-mentioned first publication in the manner which will presently be described.
A first problem is that transmission rates for the reverse channel and the forward channel in pairs are basically equal to each other. In other words, the same bandwidth is assigned to both of the reverse channel and the forward channel. However, in a case, for example, of accessing data on the internet, data in the forward direction are overwhelmingly greater in comparison with data in the reverse direction, and it results in wasting capacity of the reverse channel in a case where the reverse channel and the forward channel are equal to each other. Inasmuch as unless channel is established, the number of terminal apparatuses enable to be simultaneously connected is restricted. In addition, it jams communications for other terminal apparatuses where the frequency band overlaps.
In addition, in Paper No. B-5-35 to xe2x80x9c1997-nen Denshi Jxc3x4hxc3x4 Tsushin Gakkai Sxc3x4gxc3x4 Taikai Yokxc3x4 (1997 National Convention Record of the Institute of Electronics, Information and Communication Engineers of Japan)xe2x80x9d discloses a method of collectively transmitting data in the forward direction by using one channel. However, this method cannot be applied to the reverse direction.
A second problem is that time for reserving a channel is required.
In addition, various other publications, which may be related to this invention, are already known. By way of example, Japanese Unexamined Patent Publication of Tokkai No. Hei 7-312,783 or JP-A 7-312,783 (which will be called a second publication) discloses xe2x80x9cCDMA COMMUNICATION METHOD AND ITS DEVICExe2x80x9d each of which is capable of attaining multiplex transmission from low speed data till high speed data such as image data without much increase in the scale of the circuit. In the second publication, a basic transmission rate adopts 32 kbps higher than a usually used transmission rate of 8 kbps, the information needing a rate equal to the basic transmission rate is sent as a frame structure without an idle inter-period, and the information needing a transmission rate of 16 kbps is sent in a communication channel of frames having an idle period. Furthermore, no information is sent in an idle period other than the communication channel. Thus, the information of other channel is received for an idle period. The information at a high speed such as information whose transmission speed is 128 kbps is sent by multiplexing four channels with different spreading codes.
In addition, Japanese Unexamined Patent Publication of Tokkai No. Hei 9-055,693 or JP-A 9-055,693 (which will be called a third publication) discloses xe2x80x9cMOBILE COMMUNICATION SYSTEM AND MOBILE TERMINAL EQUIPMENTxe2x80x9d each of which is capable of providing a mobile communication system in which the throughput deterioration is not in existence by a reservation control packet by sending a reservation packet in the CDMA (code division multiple access) system. In the third publication, mobile terminals having a transmission request send a reservation packet through a reserved channel by the CDMA system and a base station allocates a transmission channel and a time slot available for the mobile terminals of request sources by using a reply packet outputted through a reply channel. A short spreading code corresponding to a matched filter is applied to the reservation channel. When the reservation packet is sent by the CDMA system, even when a plurality of reservation packets are caused overlapping timewise, the base station extracts each packet independently when a timing differs and it is not required to retransmit the reservation packet due to collision.
Furthermore, Japanese Unexamined Patent Publication of Tokkai No. Hei 9-327,073 or JP-A 9-327,073 (which will be called a fourth publication) discloses xe2x80x9cMETHOD FOR ARRANGING AND TRANSMITTING PILOT CHANNEL IN CDMA MOBILE COMMUNICATION SYSTEMxe2x80x9d each of which is capable of reducing the influence of a pilot channel on the spreading code shortage by time-dividing and multiplexing an outgoing radio channel into a plurality of time slots and assigning one of them as a pilot channel. The whole outgoing radio channels consisting of X spreading codes have frame configuration and one frame is divided into the plurality of time slots so as to be multiplexed by the division. Then, the specified time slot of the radio channel spread by the specified spreading code is assigned as the pilot channel. The other time slots and the spreading codes are used as the communication channel for communication with the mobile station. For example, time slot numbers 1-4 are given to the four time slots in the frame in order from an early one in terms of time, the time slot #1 of the radio channel where the spreading code is spread by one is assigned as the pilot channel and the other time slots and the spreading codes are assigned as the communication channel.
In addition, Japanese Unexamined Patent Publication of Tokkai No. Hei 10-013,337 or JP-A 10-013,337 (which will be called a fifth publication) discloses xe2x80x9cRADIO COMMUNICATION SYSTEM xe2x80x9d which is capable of setting the transmission power of respective stations to a proper level when a base station and a slave station carry out two-way radio communication manly by a spread spectrum system. In the fifth publication, at first, the base station transmits, to each slave station, power control data for designating the transmission output level of the slave station from its antenna together with information data, etc. Responsive to the control data, the slave station sets a transmission output based on the data and transmits it from its antenna to the base station. Responsive to the transmission output, the base station compares data indicative of its reception level with a reference level by a comparing section. When the reception level exceeds the reference level in the comparison, the transmission output is lowered by the set level. When the reception level is lower than the reference level, the transmission output is raised by the set level and power control data for permitting the transmission output level to be the same as that of the base station is transmitted to the slave station. The slave station re-sets the transmission output based on the data and re-transmits it to the base station. The above-mentioned processing are repeated so at to execute converge in the neighborhood of the reference level.
However, the second, the fourth, and the fifth publications neither disclose nor teach allocation of frequency bands suitable to transmission rates for a reverse link and a forward link. In addition, in the similar to the first publication, the third publication is disadvantageous in that time for reserving a channel is required.
It is therefore an object of the present invention to provide a CDMA communication method and a spread spectrum communication system which are capable of effectively using a frequency band.
It is another object of the present invention to provide a CDMA communication method and a spread spectrum communication system which are capable of decreasing interference in other mobile stations simultaneously connected to a base station.
It is still another object of the present invention to provide a CDMA communication method and a spread spectrum communication system which are capable of carrying out packet transmission at a shorter communication time.
Other objects of this invention will become clear as the description proceeds.
In this invention, a wide frequency band in a forward direction is shared in a plurality of mobile stations. Each mobile station monitors a signal on the wide frequency band and extracts data of its own destination from the signal. In a reverse direction, each mobile station is assigned with a channel in accordance with a necessary transmission rate.
That is, according to an aspect of this invention, a code division multiple access (CDMA) method in a CDMA system comprises base station and a plurality of mobile stations. The CDMA method comprises the steps of dividing a communication band between the base station and the plurality of mobile stations into a forward frequency band for transmitting a forward packet from the base station to each mobile station and a reverse frequency band for transmitting a reverse packet from each mobile station to the base station, and of making a forward bandwidth for use in the forward packet from the base station to a particular mobile station and a reverse bandwidth for use in the reverse packet from the particular station to the base station differ from each other. For example, the forward frequency band is used for the forward packet from the base station to each mobile station in common. In this event, the base station spreads a spectrum of the forward packet including a destination terminal identifier using a forward spreading code to produce a spread spectrum forward packet, and transmits the spread spectrum forward packet by using the forward frequency band.
In a more limited structure of the above-mentioned CDMA communication method, the reverse frequency band is divided into a plurality of reverse channels. Each mobile station is assigned with, as a selected reverse channel, one having a bandwidth corresponding to a reverse information amount of the mobile station selected from the plurality of reverse channels. Each mobile station monitors the forward packet transmitted by using the forward frequency band. A specific mobile station, which is destination of the forward packet, transmits a reply signal by using the selected reverse channel assigned to the specific mobile station with the reply signal spread spectrum using a reverse spreading code.
In a furthermore limited structure of the above-mentioned CDMA communication method, the base station preliminarily determines a plurality of forward spreading codes for use in spreading a spectrum of the forward packet, divides the forward packet in an information amount enable to transmit by using the forward frequency band using respective forward spreading codes into divided forward packets, and transmits the divided forward packets by using the forward frequency band. In addition, the base station simultaneously transmits a plurality of forward packets which are spread spectrum using different forward spreading codes. Furthermore, the reply signal which the mobile station transmits includes information indicative of a reception level of the forward packet.
According to another aspect of this invention, a spread spectrum communication system comprises a base station and a plurality of mobile stations. A communication band between the base station and each mobile station comprises a forward frequency band for transmitting a forward packet from the base station to each mobile station and a reverse frequency band for transmitting a reverse packet from each mobile station to the base station. The forward frequency band is used for each mobile station in common. The reverse frequency band comprises a plurality of reverse channels. Each mobile station is assigned with at least one of the reverse channels. The base station comprises a base station modulating arrangement for spreading a spectrum of the forward packet including a destination terminal identifier using a forward spreading code to produce a spread spectrum forward packet and a base station transmitting arrangement for transmitting the spread spectrum forward packet by using the forward frequency band.
In a more limited structure of the above-mentioned spread spectrum communication system, the base station further comprises a reply signal receiving arrangement for receiving a spread spectrum reply signal on a reverse channel assigned to the mobile station. Connected to the reply signal receiving arrangement, a reply signal demodulating arrangement despreads the spread spectrum reply signal using a reverse spreading code assigned to the mobile station. The reply signal demodulating arrangement reproduce a reply signal. Each mobile station comprises a terminal receiving arrangement for receiving the spread spectrum forward packet transmitted on said forward frequency band to produce a received forward packet. Connected to the terminal receiving arrangement, a terminal demodulating arrangement despreads the received forward packet using the forward spreading code to reproduce the forward packet. Connected to the terminal demodulating arrangement, an identifying arrangement identifies the destination terminal identifier in the forward packet. Connected to the identifying arrangement, a terminal modulating arrangement spreads a spectrum of the reply signal using a reverse-spreading code assigned to the mobile station when the mobile station has the destination terminal identifier. The terminal modulating arrangement produces the spread spectrum reply signal. Connected to the terminal modulating arrangement, a terminal transmitting arrangement transmits the spread spectrum reply signal by using the reverse channel assigned to the mobile station.
In a furthermore limited structure of the above-mentioned spread spectrum communication system, the base station has a plurality of forward spreading codes for use in spreading a spectrum of the forward packet. The base station comprises a packet dividing arrangement for dividing the forward packet in an information amount enable to transmit on the forward frequency band using the each forward spreading code to produce a plurality of divided forward packets which are supplied to the base station modulating arrangement. In addition, the base station modulating arrangement includes an adding arrangement for adding a plurality of spread spectrum forward packets to produce an added spread spectrum forward packet. Furthermore, each mobile station comprises a power measuring arrangement for measuring a reception level of the forward packet to produce a reception level measured signal. The reply signal includes the reception level measured signal. The base station further comprises a separating arrangement for separating the reception level measured signal from the reply signal. Connected to the separating arrangement and the base station modulating arrangement, an adjusting arrangement adjusts, in response to the reception level measured signal, transmission power of the forward packet for the mobile station that is prepared by the base station modulating arrangement.
Inasmuch as reverse and forward channels are assigned with frequency bands suitable to transmission rates of the reverse and the forward channels, respectively, it is possible to realize an effective use of the frequency bands. To widen a forward frequency band can cope with a high transmission rate. To share the forward frequency band with a plurality of mobile stations can restrict a necessary bandwidth in a system. Inasmuch as each mobile station monitors a forward signal of a shared band and extracts data of its own destination without reservation of a frequency band and a spreading code every packet, a communication time for reservation is unnecessary. It is possible to decrease interference between mobile stations simultaneously connected by changing allocation of forward frequency bands every mobile station. It is possible to identify a signal from each mobile station by changing spreading codes within the same frequency band.